This invention relates generally to the field of polishing apparatus and methods, and more particularly, to a chemical mechanical polishing (CMP) pad conditioning process as may be used for the manufacture of semiconductor devices.
Modern semiconductor integrated circuits are formed on layers of various materials deposited on a semiconductor wafer substrate. After layers are deposited, portions of at least some of the layers are removed by various etching processes. These manufacturing steps may result in the topography of the in-process wafer being highly irregular at certain phases of the manufacturing sequence. Such irregularities may cause problems when depositing the next layer of the structure. For example, the photolithography equipment utilized to print a pattern defining various device geometries may have a very shallow depth of focus. Accordingly, it is necessary to have a flat wafer surface in order to ensure that all portions of the pattern are in proper focus. Accordingly, it is known to polish a semiconductor wafer utilizing a chemical mechanical polishing (CMP) process in order to achieve a planar surface. Such CMP processes typically involve rubbing the surface of the wafer against a polishing device to remove high spots on the wafer surface. The wafer is generally held in a stationary or rotating fixture while being pressed against a stationary or rotating polishing pad. The polishing pad is typically supported on a platen which may be rotated by an electric motor during the CMP process. The polishing surface of the polishing pad may be an open cell foam polyurethane or a sheet of polyurethane with a grooved surface. The polishing surface is relatively rough in comparison to the semiconductor wafer surface. A slurry of polishing fluid is often introduced to further aid in the chemical mechanical polishing process.
As with any polishing medium, the surface of the polishing pad will gradually become glazed due to the accumulation of material removed during the polishing process. Accordingly, the pad must be periodically conditioned to restore its rough surface texture. Such conditioning is known to involve the application of an abrasive surface of a conditioning device against the surface of the polishing pad to remove the accumulated debris and perhaps a portion of the polishing pad surface itself. Conditioning will expose a renewed polishing pad surface having characteristics essentially the same as a new pad. Typically, a polishing pad is conditioned after each semiconductor wafer is polished.
Several types of conditioning devices are known in the art. One such device is a conditioning disk having a diamond abrasive surface formed thereon. The abrasive surface is rubbed against the polishing pad surface at a predetermined velocity, for a predetermined length of time, with a predetermined amount of force exerted therebetween. The abrasive diamond surface functions to clean, roughen and condition the surface of the polishing pad. It is known that the amount of conditioning may be varied by changing the velocity, time of contact, or force between the conditioning device and the polishing pad. The amount of conditioning will increase with an increased velocity, a lengthened time period, or an increased force between the two surfaces.
It is important to control the amount of conditioning during a conditioning operation because either too much or too little conditioning will provide undesirable results. If too little conditioning is accomplished, the surface of the polishing pad will not have achieved the desired roughness value, and the subsequent planarization or polishing of a semiconductor wafer will have a less than optimal material removal rate. Conversely, if an excessive amount of conditioning is conducted, the life of the polishing pad is unnecessarily reduced and the time spent during the conditioning process is unnecessarily lengthened. U.S. Pat. No. 5,743,784 issued on Apr. 28, 1998, to Birang, et al. describes the use of a separate floating wafer device to determine the coefficient of friction of the polishing pad in order to determine an end point of a conditioning process. As the pad is conditioned, the coefficient of friction of the polishing pad will increase, thereby increasing the horizontal sliding force exerted between the floating wafer device and the polishing pad. By measuring this horizontal sliding force, an indirect indication of the degree of conditioning of the polishing pad is obtained. Thus, variables affecting the conditioning process may be controlled in order to achieve the desired degree or rate of conditioning. While the device and method of Birang are useful in controlling a conditioning process, the cost and size of the separate floating wafer device make it a less than optimal solution.